There could be design optimizations for video (e.g. parfocal behavior) that give it a major leg up, even if its pixel-level sharpness isn't quite tip-top.
I know Canon often introduces new features in lower-end products, e.g. dual pixel AF in the 650D then 70D, but I doubt the first parfocal lens would be a low end super zoom.
Which touches my other point - the assumption is that this is a low-end zoom. Let's not forget that Canon introduced a 28-300 L - a range that was typically dismissed as "tourist class"... This lens could be a shocker in any number of ways...or not.
According to rumor, it's intended "to replace the no-so-good EF-S 18-200 IS". I don't see Canon replacing a low end lens with a high end one.
You did already. 18-135 STM IS is much better than the previous 18-135 IS. Even the 18-55 STM IS is much better than the 18-55 IS II.
Those lenses being better than their predecessors doesn't make them high end.
Furthermore, both lenses kept the same focal lengths range, which makes it easier to improve IQ.